Rapamycin, also known as sirolimus, is a 31-membered macrolide lactone, C51H79NO13, with a molecular mass of 913.6 Da. In solution, rapamycin forms conformational trans- and cis-isomers with a ratio of 4:1 (in chloroform solution) due to hindered rotation around the pipecolic acid amide bond. It is sparingly soluble in water, aliphatic hydrocarbons and diethyl ether, whereas it is soluble in alcohols, halogenated hydrocarbons and dimethyl sulfoxide. Rapamycin is unstable in solution; it degrades in plasma and in low and neutral pH buffers at 37° C. with a half-life of less than 10 hours.
Produced by Streptomyces hygroscopicus, rapamycin has been shown to possess a number of valuable pharmacological attributes. The compound is a macrocyclic triene antibiotic that possesses antifungal activity, particularly against Candida albicans, both in vitro and in vivo. See, C. Vezina et al., J. Antibiot. 28, 721 (1975), S. N. Sehgal et al., J. Antibiot. 28, 727 (1975), H. A. Baker et al., J. Antibiot. 31, 539 (1978), and U.S. Pat. Nos. 3,929,992; and 3,993,749. Rapamycin alone (U.S. Pat. No. 4,885,171) or in combination with picibanil (U.S. Pat. No. 4,401,653) has also been shown to have antitumor activity. Futhermore, R. Martel et al. Can. J. Physiol. Pharmacol. 55, 48 (1977) disclosed that rapamycin is effective in the experimental allergic encephalomyelitis model, a model for multiple sclerosis; in the adjuvant arthritis model, a model for rheumatoid arthritis; and effectively inhibited the formation of IgE-like antibodies.
The immunosuppressive effects of rapamycin have been disclosed in FASEB 3, 3411 (1989). Rapamycin, Cyclosporin A, FK-506 (also known as tacrolimus), and other macrocyclic molecules, have been shown to be effective immunosuppressive agents and therefore are useful in preventing transplant rejection. See, FASEB 3, 3411 (1989), FASEB 3, 5256 (1989), and R. Y. Calne et al., Lancet 1183 (1978). Although rapamycin shares structural homology with the immunosuppressant tacrolimus (FK506) and binds to the same intracellular binding protein in lymphocytes, rapamycin and tacrolimus have been shown to have different mechanisms of immunospressive action. Rapamycin inhibits S6p70-kinase whereas Tacrolimus inhibits calcineurin. Rapamycin was found to prolong graft survival of different transplants in several species alone or in combination with other immunosuppressants. See, S. N. Sehgal et al., Medicinal Research Reviews 14, 1 (1994).
Rapamycin is also known as an mTOR inhibitor. These inhibitors are a class of immunosuppressive drugs that inhibit T cell activation at a later stage in the immune response than other types of inhibitors like calcineurin inhibitors and DNA synthesis inhibitors. In transplantation, mTOR inhibitors are typically used in combination with calcineurin inhibitors.
Unfortunately, the side effects (e.g., gastrointestinal effects, hyperlipidemia) of mTOR inhibitors currently limit their broader use in transplantation and the treatment of autoimmune diseases. And, while not having been shown to induce nephrotoxicity, rapamycin has been shown to induce a number of toxic side effects in animal model. Such toxic effects include, for example, impairment of glucose homeostasis, gastrointestinal tract ulceration, weight loss, diarrhea and thrombocytopenia.
Numerous rapamycin derivatives have been synthesized in the hopes of alleviating and improving some drawbacks that rapamycin retains, which include low and/or variable bioavailability and solubility, and high toxicity. Mono- and diacylated derivatives of rapamycin (esterified at the 28 and 43 positions) have been shown to be useful as antifungal agents (U.S. Pat. No. 4,316,885) and used to make water soluble prodrugs (U.S. Pat. No. 4,650,803). Other derivatives include, carboxylic acid esters (PCT Publication No. WO 92/05179), carbamates (U.S. Pat. No. 5,118,678), carbonates (U.S. Pat. No. 5,260,300), amide esters (U.S. Pat. No. 5,118,678), fluorinated esters (U.S. Pat. No. 5,100,883), acetals (U.S. Pat. No. 5,151,413), silyl ethers (U.S. Pat. No. 5,120,842), bicyclic derivatives (U.S. Pat. No. 5,120,725), rapamycin dimers (U.S. Pat. No. 5,120,727) and O-aryl, O-alkyl, O-alkenyl and O-alkynyl derivatives (U.S. Pat. No. 5,258,389). Various rapamycin prodrugs have also been developed (U.S. Pat. Nos. 5,672,605, 5,583,139, 5,527,907, 5,457,111, 5,955,100, 6,146,658, and 5,935,995).
As rapamycin has been shown to possess excellent immunosuppressant, antifungal, antitumor, and other important biological activities, a need still exists for improved derivatives that increase solubility and improve the pharmakokinetic profile while decreasing its toxicity. The present invention addresses these needs.